Hiii! Not a huge Mecabricks expert, but just wanted to help. Pretty much all you can do is wait. The time interval is two weeks.
https://mecabricks.com/en/forum/topic/704/1
This is a link where someone else also wondered about this, and @Scrubbs, the creator of Mecabricks, explained.
I don't think constantly trying to change resets the interval, but just in case, now you know to wait two weeks before trying.
So, is @Bultgang the name you wished to change it to? It seems like it is, because when I went to your account, it was actually your account, not someone else's. 😉 I'm not sure why it went to someone else's, but since they are apparently not on Mecabricks anymore, perhaps you were allowed to change your username to theirs. Was their username the same as your former, unwanted username?
Anyhoo, I hope I was of some help.
-Tall

P.S. If you want to suggest a decrease in the length of time between username changes, you can make a topic in the Suggestions and Ideas category of the forums. https://mecabricks.com/en/forum/category/4
However, it seems that there is a good reason for this length of time, according to the forum in the link I related earlier.

P.P.S. For future reference, if you ever have questions like this again, simply scrolling through the help forums could provide the answers. Chances are, someone's already asked that question, or even just posted the answer on their own for users' convenience! ♡

P.P.P.S. Apparently, my brother, @SlimBrick1, says that it can take one to two weeks, and his only took one. Not sure, but there you go!

That's a lot of post scripts...

Yeah, I only had to wait one week.
(you can edit posts by the way) 😄

I know... I use that a lot... 😁
Don't tell anyone! 😄
Glad you got it figured out.

Thanks! 😄

P.S. I won't 😉

😉